The present invention relates to a method for manufacturing an optical preform including the steps of providing a substrate tube having deposited layers of glass, on the inside surface thereof, and collapsing the substrate tube by means of applying a traversing heat source to heat the substrate tube to above its softening temperature such that an optical preform is manufactured.
Generally, in the field of optical fibers, multiple thin films of glass are deposited on the inside surface of a substrate tube. Glass-forming gases (viz. doped or undoped reactive gases) are introduced into the interior of the substrate tube from one end (supply side of the substrate tube). Doped or undoped glass layers are deposited onto the interior surface of the substrate tube. The gases are discharged or removed from the other end of the substrate tube, optionally by the use of a vacuum pump (discharge side of the substrate tube). The vacuum pump has the effect of generating a reduced pressure in the interior of the substrate tube, which reduced pressure generally comprises a pressure value ranging between 5 and 50 mbar.
Several internal vapour deposition processor may be used, such as MCVD, Modified Chemical Vapour Deposition, PCVD, Plasma Chemical Vapour Deposition. The present invention is applicable to all types of internal vapour deposition processes. Hereafter, PCVD is explained in more detail.
Generally, electromagnetic radiation, preferably microwaves, from a generator are directed towards an applicator via a waveguide, which applicator surrounds a substrate tube. The applicator couples the electromagnetic radiation into the plasma. The applicator (and hence the plasma formed by that) is moved reciprocally in the longitudinal direction of the substrate tube, as a result of which a thin glass layer is deposited onto the interior of the substrate tube with every stroke or pass.
The applicator and the substrate tube are generally surrounded by a furnace so as to maintain the substrate tube at a temperature of 900-1300° C. during the deposition process.
When the number of passes increases the cumulative thickness of these thin films, i.e. of the deposited material, increases thus leading to a decrease in the remaining internal diameter of the substrate tube. In other words, the hollow space inside the substrate tube keeps getting smaller with each pass.
After the glass layers have been deposited onto the interior of the substrate tube, the substrate tube, also called precursor to a primary preform, is subsequently contracted by means of heating into an optical preform, i.e. solid rod. This is called a collapsing process. The remaining solid rod is also called a primary preform.
During the collapsing process, usually, a deposited tube (viz. a substrate tube having deposited layers of glass on the inside surface thereof) is being contracted in a contraction machine to an optical preform, utilizing a heat source. Such a machine is generally known as a collapsing device. The collapsing device comprises a heat source that can heat the substrate tube to a temperature higher than the softening temperature of said substrate tube, generally above 2000° C. Suitable heat sources for collapsing the substrate tube include hydrogen/oxygen burners, plasma burners, electrical resistance furnaces, and induction furnaces. However, the present invention is not limited to a specific type of heat source.
After said step of collapsing, in a special embodiment, the solid rod or primary preform may furthermore be externally provided with an additional amount of glass, for example by means of an external vapour deposition process or direct glass overcladding (so-called “overcladding”) or by using one or more preformed glass tubes (so-called “sleeving”), thus obtaining a composite preform called the final preform. From the final preform thus produced, one end of which is heated, optical fibres are obtained by drawing on a drawing tower. The refractive index profile of the consolidated (final) preform corresponds to the refractive index profile of the optical fibre drawn from such a preform.
From U.S. Pat. No. 4,869,743 a method of manufacturing a fibre is known by drawing a preform, wherein the preform is made by collapsing a thick-walled silica tube having a reduced pressure established therein and heated in successive passes. The tube is prevented from ovalizing by beginning each pass with a portion bearing against a coaxial internal conical guide.
From European patent application EP 0,860,719 a process and apparatus are known for monitoring and controlling the ellipticity of preform tubes during Modified Chemical Vapour Deposition. In response to computer generated signals from the monitoring device, the tube collapse rate is adjusted dynamically by locally changing the temperature of the glass tube, or by changing the physical force acting to collapse the tube.
From US patent application US 2003/0024278 a method of fabricating an optical waveguide fibre from a preform having a centerline aperture is known, which method includes reducing the pressure in the centerline aperture, then increasing the pressure in the centerline aperture to a pressure in order to improve uniformity, circularity, and/or symmetry around the centerline aperture region.
One drawback of substrate tubes having deposited layer of glass on the inside surface thereof, for example manufactured according to prior art process using an internal vapour deposition process, is that the inner diameter of these tubes is unround, i.e. not round. In other words, the cross section of the remaining “open” section of the substrate tubes does not resemble a perfect circle.
After the completion of the internal vapour deposition process, a substrate tube with deposited glass is collapsed, usually on a dedicated machine therefor. A substrate tube not being circular, for example an oval tube, will result in a non-circularity of the core of the drawn optical fibre. The above stated non-circular effect obtained during deposition is even further enhanced during the collapsing process, as during collapsing there is not only a force acting towards the centreline of the substrate tube, i.e. centrally inwards, but there is also a force acting along the circumference of the substrate tube, i.e. axially. The axial force enhances the non-circularity property of the substrate tube.
The above results in a so-called “non-circularity” of the core of the optical fibre after drawing, which is an undesired phenomenon since it leads to increased attenuation.
Non-circularity may, for example, also lead to a single mode fibre having a high polarisation mode dispersion, or may lead to a multimode fibre exhibiting high differential mode delay values due to asymmetry in the core. Both are undesirable.